1. Technical Field
The present invention relates to methods of manufacturing, and to manufacturing apparatus for, dynamic-pressure bearing devices employed in signal record/playback devices such as hard-disk drives.
2. Description of the Related Art
(1) Dynamic-Pressure Bearing Device Structures
A variety of fluid dynamic-pressure bearings have to date been employed in spindle motors used in signal record/playback devices such as hard-disk drives. Fluid dynamic-pressure bearings provide journal support by producing fluid pressure in a lubricant, such as a lubricating fluid, interposed in between a shaft and sleeve.
Single examples of spindle motors that employ a dynamic-pressure bearing of this sort are illustrated in FIGS. 10A and 10B.
The spindle motor in FIG. 10A is fit out with a dynamic-pressure bearing device 50, in which a lubricating-fluid taper seal section 53 is formed, in a single location only. The motor's shaft 51 is inserted into a sleeve 52, wherein radial dynamic-pressure bearings 55, 55 support radially directed load on the motor. Mounted on the shaft 51 at its tip is a thrust plate 56 where thrust bearings 58, 58 that bear axially directed load on the motor are formed. The bottom portion of the sleeve 52 is closed off by a thrust bushing 57, wherein the bearing gap extending from the lubricating-fluid boundary surface in the taper seal section 53 to the shaft tip is filled with the lubricating fluid, without any places in which the fluid is interrupted. The open portion of the bearing device, where the bearing gap meets the external air, is in the upper end only, and is where the taper seal section 53 is formed.
A bearing-device structure of this sort is highly reliable in that the surface area of contact between the lubricating fluid and the external air is small, and thus neither the mixing of air bubbles into, nor the gasification of, the lubricating fluid is liable to occur. Nonetheless, in order to inject lubricating fluid into the bearing device, air must be discharged ahead of time from the bearing gap, making equipment for that purpose necessary.
The spindle motor in FIG. 10B is fit out with a dynamic-pressure bearing device 5′, in which the open portions of the bearing gap are in two locations, above and below, which puts the taper seal sections 53, 53 in the two locations above and below. Although evaporation of the lubricating fluid in a bearing-device structure of this type proves to be fairly rapid, an advantage to the structure is that the central stationary shaft can be employed, for example, as a support column for supporting a hard-disk housing.
As far as the injection of lubricating fluid into the bearing device is concerned, if for example lubricating fluid is poured into the taper seal section in the upper end, it spreads along by capillary action, heading downward through the successive gap sections, and the air is discharged through the lower end. But the complex bearing-gap conformation means that there will be slight inconsistencies in the gap sections that give rise to differences in how the lubricating fluid spreads, leading to unequal permeation. Consequently, with this structure as well, it is necessary to discharge air ahead of time from the bearing gap.
In the final analysis, as long as a dynamic-pressure bearing device is not especially structured for readily discharging air from its bearing gap, when the device is to be charged with lubricating fluid, it will be necessary to exhaust the bearing gap.
(2) Publicly Known Infusing Methods and Problems Therewith
Methods such as follows are examples of techniques for injecting lubricating fluid into the bearing gap, after air filling the gap has been discharged, in dynamic-pressure bearing devices like device 50 or 5′ discussed above.
(2-1) First Method
One is a method in which the bearing device and a container filled with lubricating fluid are put into a vacuum chamber, and with the chamber in an evacuated state, the open portion of the bearing gap is either immersed in lubricating fluid or is submerged within lubricating fluid, after which air is introduced into the vacuum chamber to repressurize it. The air pressure applied in repressurization forces the lubricating fluid soundly into the full depth of the bearing gap.
Although this method may be realized with relatively simple facilities, the lubricating fluid sticks to the outside of the bearing device. Particularly in implementations in which the bearing device is incorporated into a hard disk drive, lubricating fluid having adhered to the outside of the bearing device becomes a cause of fluid contaminating the disk(s). The adhered lubricating fluid therefore must be carefully wiped off, which makes necessary a manufacturing process step that significantly impairs productivity. In implementations in which a screw-hole into which a disk clamp is fastened is provided in the head of the shaft, the lubricating fluid permeates the screw-hole and the thread groove. Removing lubricating fluid that has permeated a narrow area of this sort in the bearing device is extremely difficult.
(2-2) Second Method
An alternative technique is a method in which the bearing device is set inside a vacuum chamber, and with the chamber in an evacuated state a cylindrical capillary tube such as a fine syringe needle is used to trickle lubricating fluid into the open portion, or the taper seal section, of the bearing device, following which the chamber is repressurized.
Employing this method might lead to the expectation that the process step for wiping away lubricating fluid that has stuck to the outer side of the bearing device could be omitted, but in actuality the method does not necessarily work well. This is because when the lubricating fluid is squirted from the needle tip, frequently the fluid froths at the tip and the froth bursts, splattering on and contaminating the outside of the bearing device.
It might then seem that a way to get rid of the frothing would be beforehand to sufficiently clear the lubricating fluid of air that has dissolved into it. In practice, however, frothing occurs even if the lubricating fluid undergoes a degassing process, such that contamination of the bearing device exterior is eliminated only with difficulty.
Furthermore, with the method in which a syringe needle is used, the amount of lubricating fluid that will just fill the bearing gap has to be measured accurately and trickled (or injected) into the gap. With the foregoing first method, because the amount of lubricating fluid required to fill the bearing gap is forced into the bearing device during repressurization, the bearing gap is always left having been filled up with lubricating fluid, so that there is never a shortfall. And since any amount that overflows from the taper seal section 53 is wiped up together with the rest of the lubricating fluid, there is also never a surplus. Advantage cannot be taken of such phenomena and wiping-up work in the case in which lubricating fluid is trickled into the bearing device.